1. Field of the Invention
The present invention relates to semiconductor light-emitting devices and methods for making such devices. More particularly, the invention relates to a short wavelength semiconductor laser which emits light beams of red to orange colors and a light emitting diode (LED) having a wavelength band of red to green colors, and methods for making the same.
2. Description of the Prior Art
Recently, red semiconductor lasers having a wavelength band of 630 to 680 nm constructed of AlGaInP have been receiving attention as a promising source of light for POS, as well as for high definition, high density photomagnetic disks. Indeed, researches and developments have been made in this connection. When such laser is used for disks, fundamental mode stability and good optical characteristics such as astigmatism and so on, in particular are important. For this reason, there exists a need for a semiconductor laser of the refractive index guide type which confines light beams within the region of oscillation.
Refractive index guide type semiconductor laser devices of 680 nm wavelength band have hitherto been known including one shown in FIG. 11 which is of the effective refractive index guide type, and another shown in FIG. 12 which is of the real refractive index guide type. FIG. 11 is a sectional view showing the semiconductor laser device of the effective refractive index guide type, and FIG. 12 is a sectional view showing the semiconductor laser device of the real refractive index guide type. The semiconductor laser device shown in FIG. 11 is fabricated in such a way that on an n-GaAs substrate 131 having (100) face as a main face are grown an n-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P cladding layer (1.5 .mu.m thick) 133, a non-doped Ga.sub.0.5 In.sub.0.5 P active layer (0.05 .mu.m thick) 134, a p-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P cladding layer (1.5 .mu.m thick) 135, and a p-Ga.sub.0.5 In.sub.0.5 P intermediate layer 136 according to a MOCVD (Metal Organic Chemical Vapor Deposition) process. Then, the intermediate layer 136 and an upper portion of the cladding layer 135 are removed by etching, leaving a centrally located ridge portion 141. Subsequently, n-GaAs current constrictive layers 132 are grown on both sides of the ridge portion 141 and, in addition, a p-GaAs contact layer 137 (2 .mu.m thick) is grown over the entire region. Finally, electrodes 139, 140 are formed respectively on the underside of the substrate 131 and on the surface of the contact layer 137. In such a semiconductor laser, the current constrictive layers 132 limit current passage to decrease ineffective current and cause a substantially large mode loss relative to a higher order mode of oscillation. Thus, an oscillation mode of higher order is suppressed so that oscillation of the fundamental mode is steadily maintained in oscillation region 134a to a high light output.
The semiconductor laser device shown in FIG. 12 is fabricated in such a way that on a p-GaAs substrate 101 having (100) face is formed an n-GaAs current constrictive layer 102 in which a channel 102b is formed reaching from the surface of the layer 102 into the substrate 101. Then, a p-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P cladding layer (1.8 .mu.m thick) 103, a non-doped Ga.sub.0.5 In.sub.0.5 P active layer (0.05 .mu.m) 104, an n-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P cladding layer (1.5 .mu.m thick) 105, and an n-Ga.sub.0.5 In.sub.0.5 P contact layer (1 .mu.m thick) 106 are sequentially formed over the layer 102 according to the MOCVD process (where each layer thickness value denotes the thickness of the respective layer in the channel). Finally, electrodes 109, 110 are formed respectively on the underside of the substrate 101 and on the surface of the contact layer 106. In the stage of growth according to the MOCVD process, the layer being grown usually reflects the configuration of the base layer. Therefore, the active layer 104 is of such a configuration that it is largely bent above the edge of the channel 102b, that is, above corresponding ends of the current constrictive layer 102, whereby a real refractive index guide structure is formed. According to this arrangement, possible loss in the fundamental mode is reduced, which results in reduced threshold oscillation value and increased differential efficiency.
Unfortunately, however, the semiconductor layer shown in FIG. 1 involves a problem that the stability of the fundamental mode depends largely on the thickness (residual thickness) d of the cladding layer portions remaining at both sides of the ridge 141. This means that when etching variations are so wide that the residual thickness d substantially exceeds 0.3 .mu.m, the fundamental mode is rendered unstable. (It is noted in this connection that an optimum value of residual thickness d is approximately 0.2 .mu.m.) The same is true with the case in which residual thickness d differs on opposite sides of the ridge 141. Another problem is that in the stage of current constrictive layer 132 growing, the base layer for such growth is a layer including Al, that is, the p-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P layer 135, which fact is likely to be a cause of oxidation so that the quality of a regrown interface 135a will be unfavorably affected. This results in current leaks which in turn lead to an increase in the oscillation threshold value. Typically, a non-coated device having a resonator length of 400 .mu.m has an oscillation threshold value of 45 mA and a kink level of about 25 mW.
The semiconductor laser device shown in FIG. 12, wherein the active layer 104 is largely bent at ends of the current constrictive layer 102 to provide a real refractive index guide construction, has smaller losses in both fundamental and higher order modes. This presents a problem that the kink level becomes rather lowered. Typically, a non-coated device having a resonator length of 400 .mu.m has an oscillation threshold value of 25 to 30 mA and a kink level of about 20 mW.